The present invention relates to a method and a device for adaptive control of cutting machining operations by using quantities measured during the machining and characteristic of the productivity optimizing of the machining operation.
The object of the adaptive control intended by the invention is to maximize productivity. In cutting machining operations, which are the main object of the present invention, the productivity can be expressed as the inverted value of the cost per volume unit of machined material. When so defining the productivity, it is understood that a desired exactness of dimensions must be achieved and that the machined surface must not be inferior to what is desired. Moreover, it is understood that the machining is performed in a given machine with given tools and that the upper limits of force, power, moment and cutting velocity demanded by the combination machine-tool-working piece must not be exceeded. Thus, for adaptive control, a limited number of independent variables are available, and the productivity is regarded as a function of only these independent variables.
In order to describe the method of adaptive control in stock removal, a turning operation, in a numerically controlled machine is chosen. When turning, one has, in the simplest case, to work with only two independent variables, viz, feeding and cutting speed. In more sophisticated systems, a third independent variable may be introduced for instance, the cutting depth.
In stock removal, the tool is worn and after a certain time T it is worn-out and must be replaced. The cost of the tool and the time for replacing the tool must be considered. The wearing time T is therefore of considerable importance to the productivity. According to Colding [e.g. Annals of the C.I.R.P. (International College for Production Engineering Research), volume 17, 1969, pages 279-288], the productivity can be expressed as ##EQU1## WHERE V = cutting speed, q = chip equivalent (incorporating, in a single parameter a plurality of cutting parameters), T = tool-life and T.sub.v = tool cost (including the costs associated with tool changin, and other related factors affecting tool costs), all converted into machine time. It is, of course, well-known in the art that the chip equivalent concept is applicable not only to turning operations but also to milling and grinding. Thus, chip equivalent may be expressed by the formula q = L/(T.S) where L is the whole cutting edge length, t is the cutting depth, and S is the feed rate. As is known in the art, however, chip equivalent may also be expressed in a more general way by the formula q = L/A, where L equals the engaged cutting edge length and A equals the cross-sectional area of the underformed chip.
What has so far been said has been known for a long time and many efforts have been made to calculate in advance the tool-life T in order to make possible calculations of cutting values giving maximum productivity. Taylor3 s equation V (T.alpha.)= constant, where V is the cutting speed and .alpha. is a material constant, is also known. This equation, however, has a limited range of validity, which has been shown by Colding among others. Furthermore, it does not consider feed rate, cutting depth or chip equivalent, which quantities have a great influence on tool-life.
From a general point of view, it can be said that all calculations to predict tool-life T give very uncertain results, if one does not check during the machining, that the conditions of the calculations are valid.
In adaptive control, T can be calculated with a greater exactness by continuous or intermittent measurement during machining of quantities having decisive importance on T.
In the literature, several attempts at calculating T by using formulas containing measured cutting forces, powers and cutting edge temperatures are reported. The results, however, have been negative from a general point of view.